Autodeposition has been in commercial use on steel for about thirty years and is now well established for that use. For details, see for example, U.S. Pat. Nos. 3,063,877; 3,585,084; 3,592,699; 3,674,567; 3,791,431; 3,795,546; 4,030,945; 4,108,817; 4,178,400; 4,186,226; 4,242,379; 4,234,704; 4,636,264; 4,636,265; 4,800,106; and 5,342,694. The disclosures of all these patents are hereby incorporated by reference. Various resin systems have been used including vinyl, acrylic, epoxy and hybrid polymer systems. Epoxy resin and epoxy-acrylic autodeposition coating systems are described in U.S. Pat. Nos. 4,180,603; 4,289,826; 5,500,460; 6,096,806, 6,989,411, 7,138,444, and 7,388,044 and International Publication Number WO 00/71337, the teachings of each of which are incorporated by reference.
Autodeposition compositions are usually in the form of a liquid, usually aqueous solutions, emulsions or dispersions in which active metal surfaces of inserted articles are coated with an adherent resin or polymer film that increases in thickness the longer the metal remains in the bath, even though the liquid is stable for a long time against spontaneous precipitation or flocculation of any resin or polymer, in the absence of contact with the active metal. Those of skill in the art will understand resin as an organic polymer, most often synthetic, of at least about 5000 Daltons. When used in the autodeposition process, the autodeposition composition when cured forms a polymeric coating. “Active metal” is defined as metal that spontaneously begins to dissolve at a substantial rate when introduced into the liquid solution or dispersion. Such compositions, and processes of forming a coating on a metal surface using such compositions, are commonly denoted in the art, and in this specification, as “autodeposition” or “autodepositing” compositions, dispersions, emulsions, suspensions, baths, solutions, processes, methods or a like term. Autodeposition is often contrasted with electrodeposition. Although each can produce adherent films with similar performance characteristics, the dispersions from which they are produced and the mechanism by which they deposit are distinctly different. Electrodeposition requires that metal or other articles to be coated be connected to a source of direct current electricity for coating to occur. No such external electric current is used in autodeposition.
More recently attempts have been made to autodeposition coat non-ferrous metals such as zinc, aluminum magnesium and alloys thereof. Defect problems arose in seeking to use a single autodeposition coating bath to coat two or more different metal surfaces, such as ferrous, zinc, aluminum, magnesium and alloys thereof due to different activity levels of the metals, which required changes to bath chemistries even when coating different substrates at different times. It would thus be beneficial to produce an autodeposition coating composition and/or bath that can be used to coat two or more of these dissimilar metal surfaces, either simultaneously or sequentially, without significant changes in bath chemistry being required.
Another area where improvement to autodeposition coating compositions would be beneficial is in coating combination multi-metal substrates, which can be a metal substrate comprising a coating of a dissimilar metal, e.g. galvanized or galvannealed metal; assemblies of two or more dissimilar metal parts, such as a door of a passenger car; or a combination thereof. Conventional autodeposition coatings applied to a multi-metal workpiece had some shortcomings, resulting in pinholes and poor edge coating on metal workpieces comprising layers of dissimilar metals in the area of the exposed interface of two metals, for example at a cut edge of a metal panel coated with a dissimilar metal. Pinholes will be understood by those of skill in the coating arts to be small voids, about 0.1 to 1 mm in diameter, in the autodeposition coating where the coating's coverage is much thinner than the surrounding area. Some, but not all, pinholes may expose the metal substrate surface. These pinholes have been identified as corrosion initiation sites to be avoided.
In attempting to eliminate pinholing in autodeposition coatings on multi-metal substrates, methods and compositions have been discovered which have reduced the pinholing effect on some substrates, even though subject to somewhat sensitive process parameters and tight tolerances for the coating process. Conventionally, in order to coat the different metal substrate materials in one bath, a high level of hydrogen peroxide is used in the autodeposition bath in order to accomplish a satisfactory coating, as described in co-owned United States Patent Application Number US 2008/0160199A1, which is incorporated herein by reference in its entirety, to the extent not contradicted herein. Maintenance of high levels of peroxide in an autodeposition coating bath has some shortcomings. Conventional autodeposition coating of workpieces that have metallic substrates with different activity levels has required a very careful composition selection to achieve a defect free coating on all metals presented in the bath together. Although the tight window of success has made manufacturing possible, it has also made manufacturing more difficult, costly and time consuming. Broadening the processing tolerances is desirable for providing a higher degree of success in manufacturing thereby solving problems of re-work and scrap rate. Simplicity in coating process design and control is critical to providing value to a customer.
Highly active zinciferous metals like hot dipped galvanized, electro-galvanized or galvanneal substrates required maintenance of high levels of peroxide in the autodeposition bath to coat in a defect free manner, meaning no pinholes. Maintenance of high levels of peroxide is difficult due to self decomposition of the peroxide and wide fluctuations of the concentration during production. Even at these high peroxide levels, a few pin-holing types of defects were still visible on highly active substrates unless the peroxide concentrations are very carefully monitored to maintain them within a narrow operating concentration. Under such high peroxide concentration conditions, it is increasingly difficult to satisfactorily autodeposition coat the entire panel uniformly, complete with coatings on the raw, uncoated edges of a stamped panel. Each of the various metals, ferrous metal, hot dip galvanized, electro-galvanized or galvanneal substrates requires different concentrations of hydrogen peroxide to provide satisfactory coatings using autodeposition baths. As such, to autodeposition coat two or more of these metal substrates in a single bath under these conditions becomes even more difficult. Attempts to simultaneously coat two or more such substrates with a uniform coating have not been completely successful. Edge coating is problematic and is not always successful. In a manufacturing setting, where process control of large baths over time is very challenging, there is a strong need for new chemistries that solve the problems of pinholing and edge coverage, without the need for maintenance of high peroxide and stringent control of such parameters.
Even in a high peroxide autodeposition bath, difficulties still remain in obtaining a uniform autodeposition coating deposited in the same bath at the same time on the entirety of two dissimilar metal surfaces, for example on the steel and the zinciferous or aluminiferous metallic surfaces of a workpiece. Edge coverage is a particular problem. The narrowest edges and protuberances of a workpiece, as well as interfaces of two dissimilar metals, for example cut edges of a ferrous workpiece coated with a non-ferrous metal, e.g. zinc, zinc alloy, aluminum or aluminum alloy, still require improvement in autodeposition coating coverage. Because a unique feature of the autodeposition process is the formation of a uniform film over the entire surface of the work piece, even in difficult-to-reach areas, it is desirable that uniform coatings on multiple metal containing substrates be more easily achieved.
Therefore, for many reasons, autodeposition coating of multiple metal substrates in their manufacturing process could be made much simpler if a more forgiving autodeposition bath would be available that would uniformly coat combination multi-metal substrates, such as for example, zinciferous galvanized panel faces along with un-coated ferrous metal substrates.